The most contagious travel illness news of late was, of course, all about the swine flu. But good old Montezuma’s Revenge — and in no way does that refer only to the Mexican strain of travel-induced diarrhea — is by far the most common travel illness threatening a carefree vacation.

“It’s very hard to avoid. About 40 percent of people will get diarrhea when traveling in an undeveloped country, which covers most of the world,” says Dr. Stephen Berger, founder and medical advisor for GIDEON, on online infectious diseases database. Exposure to different strains of the E. coli bacteria, present in all of our bodies, says Berger, is what usually causes diarrhea in travelers, and it can happen in any country.

“Americans are very insular and can see these conditions as other people’s diseases, hence the term Montezuma’s Revenge,” says Berger. “But when Mexicans go to America they get diarrhea, too, since they’re being exposed to different bacteria.”

And while most travelers know the ABCs of staving off these unfortunate trips to the restroom — l ike avoiding street fruit and vegetables, sticking to bottled water, eating thoroughly cooked foods, replacing lost fluids with electrolyte drinks, to name a few — not all travel illnesses come with such clear guidelines for steering clear of trouble.

Case in the point, the dreaded norovirus, which has incapacitated an onboard party on more than a few cruise ships in recent years and, unfortunately, has no hard and fast measures for prevention and treatment.

Kristyn Wilson, a media relations manager in Ohio, was aboard a Royal Caribbean cruise ship in 2006 when the norovirus broke out.

“My grandfather took the entire family on a cruise, there were about 20 us,” she recalls, “It was the trip of a lifetime for the whole family, and kind of like my grandpa’s last hurrah.”

Wilson says that one of her cousins was among the first to fall ill. After visiting the ship’s doctor, Wilson’s cousin was diagnosed with the norovirus and promptly quarantined in her room, along with another member of the family who was sharing the same cabin.

“For the second half of the trip, we basically didn’t see them anymore,” says Wilson of her sickened family members. “They had to stay in their cabin and the crew would deliver them food wearing plastic gloves and face masks. When room service would come to the door, you would think we passengers had ebola!"

“There’s no real prevention and no treatment, and the cruise ship industry is going crazy with how to control it,” says Dr. Berger of the norovirus. “Some have to pull back into port and use brutal sterilization of the water system and tanks holding the water, [and] then they’ll have another outbreak.”

Moving from waterborne illnesses to the insect-spawned variety, there’s a bevy of mosquito bite-induced maladies threatening travelers in to the world’s tropical regions — and beyond. It used to be that malaria was only a problem in tropical areas, but in recent years, says Dr. Berger, outbreaks have occurred in places such as Marseilles, France and Florida.

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“And with the chikungunya outbreak in Italy in 2007, that’s not tropical any more either,” says Dr. Berger, referring to another mosquito-borne illness that manifests with terrible joint pain, muscle pain, a fever, and often a rash. An outbreak in Ravenna, Italy, in 2007 showed the disease was spreading beyond the tropical realm. “The mosquitoes that cause these things turn out to be mosquitoes that also invade non-tropical areas,” says Berger, “Chikungunya is extremely common in the Indian Ocean region, and it’s becoming more and more common to see it in travelers in the U.S. and Europe, too.”

Cases of dengue fever are also on the rise around the world, says Joan Hersh, a registered nurse at the travel clinic at St. Elizabeth Business Health Center in Edgewood, Ky., who regularly advises missionaries on travel health issues before they depart for far flung locales. “Before, it was unusual to get it [dengue fever] during short trips,” says Hersh, “But now tourists and travelers are getting it. In tropical areas, we see a lot of dengue right now.”

The incidences have been particularly high in patients returning from Central America and India, she says, but reports show that there have been dengue cases all over the Caribbean as well. Precautions for avoiding dengue fever are the same as with all mosquito-borne illnesses: Wear long sleeves and pants at sunset and at times when bugs are known to bite, be generous with the insect repellent and sleep in a screened-in room with a mosquito net as an extra layer of protection.

Unlike with malaria, there are no preventative pills you can take to stave off the illness. And unfortunately, if you're affected, there's no specific treatment for dengue, according to the CDC's (Center for Disease Control and Prevention) Web site, which advises patients to consult a physician, drink plenty of liquids and get lots of rest.

Patients need to be their own advocates, Hersh emphasizes, as travel-related diseases sometimes don’t manifest until long after the travelers have returned home. It’s important to keep your doctor apprised of where you’ve been and where you’re going, says Hersh, as even something as seemingly innocent as a white water rafting trip in Costa Rica can result in a water-borne sickness.

“One of the biggest problems with foreign travel that we see is that some of these illnesses are so unusual here in the U.S. that, most of the time, physicians aren’t even looking for them,” Hersh says. “So getting a diagnosis is very hard.” Misdiagnosed illnesses can linger, she says, and get worse.

“Patients need to make sure they are telling their doctors where they’ve been, really volunteer that information,” Hersh urges. “You can save yourself a lot of grief.”